In the past, having multiple webpages open meant having your desktop cluttered with browser windows. Modern browsers give you the option to quickly switch between pages with tabs instead of separate windows.
If you hold Ctrl when clicking a link, a new tab will open the page while the original page remains open.
If you hold Shift when clicking a link, the page will open in a new browser window. If you use multiple monitors, this allows you to drag the new window to a different monitor and see both pages at once.
Tabs in Chrome
Tabs in Firefox
Tabs in Internet Explorer 11